Wind energy, as a major sustainable clean energy source, has recently attracted an intense amount of attention. Government and industry have aggressively pushed the development of wind turbine technology, particularly devoted to the development of a wide range of reliable wind turbines.
In the long history of the use of wind kinetic energy, horizontal-axis wind turbines (HAWT) and vertical-axis wind turbines (VAWT) have been adopted as the main designs for extracting wind energy. In particular, HAWT is predominantly used for commercial power generation. The efficiency of turbine blades has improved considerably over the years by the advent of advanced aerodynamic designs. However, the basic law that aerodynamic force is proportional to the wing area of a blade remains unchanged. Thus, it is difficult to maximize the effective aerodynamic surface of rotating blades, while maintaining the structural integrity thereof, as the size of a wind turbine increases.
For medium and large size wind turbines, to reduce the centrifugal force in rotational motion, blades are usually long and slender particularly near the tips even though they are made of light composite materials. Thus, the effective aerodynamic area is very limited near the tip of a blade where a considerable amount of aerodynamic torque is generated. In addition, to further improve the aerodynamic efficiency, a mechanical-electrical device for active pitching can be installed inside of a slender blade. This arrangement not only adds a weight penalty on a slender blade, but also increases the structural complexity and weakens the structural integrity of the blade.
Furthermore, the installation and maintenance of long slender blades in large-size HAWT and VAWT is particularly difficult, and requires special equipment that may not be easily operated in certain terrain. For example, contamination of dead bugs and birds near the leading edges of blades could reduce the aerodynamic efficiency by 50%. Cleaning the contamination on blades of a large-size HAWT is not an easy task. These problems become bottlenecks in the development and cost-efficient use of medium and large size wind turbines. Improvements in existing water-powered generators are also desirable.